1. Field of the Invention
The present invention relates to a novel dinuclear metallocene complex, and more particularly to a novel metallocene complex that is suitable for preparing olefin polymers having a high molecular weight.
2. Description of the Prior Art
Olefin-based polymers have been used in a wide range of applications. One group of commonly used olefin-based polymers are polyolefins, that is, homopolymers or copolymers of olefins. These polyolefins plastics are typically used in such applications as blow and injection molding, extrusion coating, film and sheeting, pipe, wire and cable.
Most of the physical and mechanical properties of polyolefins, such as their high strength and high impact, stress, and puncture resistances, together with their high toughness, are attributed, at least in part, to their relatively high molecular weight.
An example of polyolefin is ethylene-propylene elastomer (ethylene-propylene rubbers, EPM). It finds many end-use applications due to their resistance to weather, good heat aging properties and their ability to be compounded with large quantities of fillers and plasticizers. Typical automotive uses are radiator and heater hoses, vacuum tubing, weather stripping and sponge doorseals. Typical industrial uses are sponge parts, gaskets and seals.
Another group of commonly used olefin-based polymers are terpolymers of ethylene, propylene, and a non-conjugated diene, which are generally referred to as EPDM elastomers. EPDM elastomers have outstanding weather and acid resistance, and high and low temperature performance properties. Such properties particularly suit EPDM elastomers for use in hoses, gaskets, belts, and bumpers; as blending components for plastics and for tire side walls in the automotive industry; and for roofing applications. Additionally, because of their electrical insulation properties, EPDMs are particularly well suited for use as wire and cable insulation.
In order for EPMs and EPDMs to have commercially acceptable properties, they should have a Mooney viscosity at 127.degree. C. no less than 10, a weight-average molecular weight no less than 110,000, a glass temperature below -40.degree. C., and a degree of crystallinity no greater then 25%.
However, to date, EPMs and EPDMs obtained by catalysis of metallocenes usually have a weight-average molecular weight of 100,000 or less, which does not meet the commercial requirements for an elastomer.
In recent years, it has been found that polyolefins having a multimodal (typically bimodal) molecular weight distribution (MWD) will not only retain the advantageous properties associated with high molecular weight, but also exhibit improved processability.
A bimodal MWD polymer (which can be also simply referred to as "bimodal polymer") is defined as a polymer having two distinct molecular weight distribution curves as observed from gel permeation chromatography (GPC). In other words, a bimodal polymer can be thought of as a mixture containing a first polymer with a relatively higher molecular weight blended together with a second polymer with a relatively lower molecular weight.
Various approaches have been disclosed for producing bimodal polyolefins. The simplest approach is to physically blend together two polymers having different molecular weights. However, this simplistic approach suffers from the problem that homogenization can be obtained only with polymers that can be completely molten. If one of the polymers is not completely molten, then the polymer blend will be inhomogeneous. This can cause a myriad of problems.
U.S. Pat. Nos. 5,284,613 and 5,338,589 disclose a two stage polymerization process for preparing a bimodal polyolefin. In the first stage, olefin monomers are contacted with a catalyst under polymerization conditions to produce a relatively high molecular weight (HMW) polymer, wherein the polymer is deposited on the catalyst particles. In the second stage, the HMW polymer containing the catalyst is further polymerized with additional olefin monomers to produce a relatively low molecular weight (LMW) polymer, much of which is deposited on and within the HMW polymer/catalyst particles from the first stage. The disadvantage of such a two stage process is that two reactors are needed, resulting in an undesirably high capital investment.
U.S. Pat. No. 5,369,194 discloses a process for preparing bimodal polyolefins in a single reactor. The catalyst system so used includes two different transition metal catalysts supported on the same solid support material. Therefore, high and low molecular weight polymers can be formed on the same catalyst particle. The shortcoming is that procedures for preparing the solid support material supporting two different catalysts is complicated and difficult. Moreover, the preferable conditions for activation of the two catalysts may be different. Therefore, when one catalyst is activated, the other catalyst may be inactivated.